This invention relates to a toroidal continuously variable transmission and a method for producing torque a transmitting member thereof.
In a toroidal continuously variable transmission for an automotive vehicle etc., a plurality of rollers are rotatably sandwiched between raceway portions of an input disc and an out put disc opposing each other by a predetermined force. Such a toroidal continuously variable transmission transmits torque between the discs by a traction force between the circumference of the roller and each raceway portion of the disc. This toroidal continuously variable transmission can increase and decrease the number of revolutions of the output disc against the input disc (transmission ratio) by adjusting the position of the roller with respect to each disc.
In the toroidal continuously variable transmission, each of rolling contact portions of the input disc, the output disc and the rollers, which are torque transmitting members, suffers high temperatures and high surface pressure in operation. For this reason, not only a black structure caused by conventional fatigue but also a white structure causing white layer flaking is occurred at the rolling contact portions. This causes a problem that its life is shortened. Some technologies trying to solve such a problem are known as follows:    (1) In order to increase fatigue resistance, the production process comprises performing carburization treatment on a mechanical structural steel containing Cr so that the material member has a surface carbon density within a range of 0.6-1.5% by weight; precipitating carbide by holding the carburized material member at an upper limit temperature which is calculated based on the alloy content; and quenching after holding at a temperature at which austenite phase is formed, and the mechanical structural steel includes a matrix having at least one of martensite structure and bainite structure. The matrix contains carbide, which includes M23C6 type carbide, having a mean particle size of 3 μm or less, dispersed and precipitated in form of at least one of generally spheres and pseudo-spheres (see, for example Japanese Patent Laid-Open Publication No. 2001-98343).    (2) In order to reduce the occurrence of the structural change, the amount of residual austenite in steel is set to 10% or less (see, for example Japanese Patent Laid-Open Publication No. 1996-210360).    (3) In order to reduce a temperature rise by reducing rolling friction, the amount of residual austenite in steel is set to not less than 0.05% and less than 10%. This also tries to prevent a plastic deformation caused by the decomposition of residual austenite at a rolling contact surface (see, for example Japanese Patent Laid-Open Publication No. 1997-105415).    (4) An alloy steel material, which contains C: 0.15-0.5 wt %, Si: 0.1-1.5 wt %, Mn: 0.1-1.5 wt %, Cr: 0.5-3.0 wt %, and Mo: 0.1-3.0 wt %, a content of O in the alloy steel material being set to be not more than 9 ppm, which surface after carbonitriding, hardening and tempering treatments is set to contain C: 0.8-1.2 wt % and N: 0.05-0.20 wt %, and which surface hardness is Hv 720 or more, and in which Mo or V carbide/carbonitride, the average particle size of which is in a range of from 50 nm to 500 nm, is dispersed and precipitated on a race surface or in a range of from the race surface to a deep position where a maximum shearing stress is generated, is used (see, for example Japanese Patent Laid-Open Publication No. 2001-32900).
However, there is a problem that any of the technologies mentioned above does not sufficiently achieve the effect and its cost is high.
It is an object to provide a toroidal continuously variable transmission and a method for producing a torque transmitting member thereof capable of long life at low cost.